Field
Various communication systems may benefit for techniques and devices for improved connection reliability and interference management. For example, communication systems of the long term evolution (LTE) of the third generation partnership project (3GPP) or LTE advanced (LTE-A) may benefit from systems and methods for improving connection reliability and managing interference created by beamforming.
Description of the Related Art
In an LTE or LTE-A network, before a user equipment can start calls or data traffic with the network, the user equipment (UE) has to listen for certain broadcast information from a network element, such as an evolved Node B (eNB). Call setup success rate is key performance indicator (KPI) for a deployed network. For reliable call setup and access, the access procedure should itself be highly reliable.
In LTE, like other cellular technologies, the call setup procedure involves a series of steps as follows: UE acquires the LTE network; UE decodes the system information broadcasted to all UEs within the cell using a broadcast control channel (BCCH) which provides the UE with the basic information it needs to know how to access this eNB/network; UE sends a random access request (RACH) to the eNB (MSG1); eNB responds to the RACH with MSG2 (Random Access Response) that includes UL grant allowing the UE to transmit a connection request, Temporary C-RNTI and timing advance value; using the UL grant that arrives with MSG2, UE sends RRC connection request to eNB (MSG3); eNB responds with connection setup using the Physical Downlink Shared Channel (PDSCH) (MSG4 or Contention Resolution Message); eNB configures other aspects of the UE, such as configuring the transmission mode, other channel attributes and parameters such as the Sound Reference Signal (SRS) configuration to use, additional radio bearers to use, handover parameters and thresholds, etc.; and UE is now connected and able to send and receive user data using the Dedicated Traffic Channel (DTCH).
In order to be spectrally efficient and support large data throughputs, the 3GPP specification has defined many advanced multi-antenna transmission modes. In TD-LTE, transmit modes 7 and 8 (TM7 and TM8) employ beamforming techniques. These advanced transmission modes attempt to drive RF signals from multiple antennae at the eNB with different amplitudes and phases so that these signals combine coherently at the UE receiver. The amplitude and phase applied to each antenna element is called the beamforming weight. With a properly chosen beamforming weight, the UE receives his desired signal at a much higher signal strength compared to the noise floor and other interference sources. The ratio of desired signal power to the sum of noise and interference power is referred to as the Signal to Noise plus Interference ratio (SINR). A high SINR is viewed as indicating good channel quality, which is required for either link—uplink (UL) or downlink (DL)—to support high throughput transmissions.
In TD-LTE, due to the reciprocity assumption between the DL channel and UL channel, the eNB can compute the beamforming weight to be applied to a specific UE from the UL pilot symbols that the UE is configured to transmit periodically. The UL pilot symbols that are transmitted by the UE for the purpose of sounding the channel are called Sounding Reference Signals (SRS). Once the eNB receives the SRS from the UE, the eNB is able to form the channel covariance matrix and perform an eigenvalue decomposition of the channel covariance matrix to determine the ideal beamforming weights to use for that particular UE. Beamforming has the ability to concentrate and coherently combine the transmitted signal from multiple eNB antennae to get a higher received signal strength.
In order to calculate the special beamforming weight to a particular UE, the UE first needs to be connected to the eNB and configured to transmit the SRS signal. Therefore, until the UE attaches to a particular cell, it receives all the broadcast information in a non-beamformed transmission mode that uses a broad sector beam. In a dense TD-LTE network deployment that employs TM7 and/or TM8, when a UE close to the boundary between two cells attempts to connect to a cell, it is likely to be heavily interfered with by DTCH transmissions from an adjacent cell that is employing beamforming. The resulting interference can lower the SINR of the broadcast signal and common control channel to such a large extent that the UE may not be able to connect to the desired cell. This reduces the connection reliability and call setup success rates to unacceptable levels.
As an example, FIG. 1 illustrates the situation that can occur. Specifically, FIG. 1 illustrates an attach challenge for a user equipment in adjacent cell. As shown in FIG. 1, UE#1 in Cell 1 may be operating in TM7/8 mode, with a UE-specific beam pattern. This beam pattern may interfere with the broadcast signal in Cell 2 to UE#2, which is trying to attach to Cell 2.
The interference issue can be exacerbated when the eNB antenna that is deployed has antenna spacing larger than 0.5λ, where λ is the wavelength corresponding to the carrier frequency used. When the spacing is larger than 0.5λ, large grating lobes may occur, that can produce a lot of interference in adjacent cells.
FIG. 2 illustrates interference in adjacent cell due to grating lobe. An example of the grating lobe issue can be observed in FIG. 2. For example, when Cell#1 is using beamforming weights directed towards a user at 50° from the boresight of the antenna array, a large grating lobe can be produced at −45° causing approximately 8-10 dB of interference to UEs at the edge of Cell#2.